[POWERPC] windfarm: Fix windfarm thread freezer interaction

When I fixed the windfarm freezer interaction first in commit
1ed2ddf3, an earlier patch than the one
I came up with after comments was committed. This has come back to haunt
us now because commit d5d8c597 changed
the freezer to no long send signals. Fix it by removing the windfarm
thread's signal logic and restoring the original try_to_freeze().

We could simply revert 1ed2ddf3

 now
but I feel that the assertion that no signal is delivered to the
windfarm thread needs not be there.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Paul Mackerras <paulus@samba.org>